1. Field of Invention
This invention relates to improvements in the uniformity of air core coil magnets used in nuclear magnetic resonance (known as NMR) imaging devices.
2. Description of the Prior Art
An NMR imaging apparatus obtains a picture of a selected plane in the body of a subject being examined, without invading the body. In this kind of an NMR apparatus, an air core coil is used for the purpose of producing a magnetic field, more particularly a static magnetic field.
Generally, as depicted in FIG. 1, when an air core coil is wound in a single pancake or a double pancake, a wire is wound around a bobbin 1, which may be cylindrical or columnar. Toward the end of the first turn of the wire, a gap 3 exists between the bobbin 1 and the wire. As a result, every succeeding turn of the wire fails to form a completely round surface. (Since the degree of uniformity required for a magnet for NMR is on the order of 10.sup.-5, such a coil can not be handled as a circular ring coil and some corrections are necessary.)
FIG. 2 shows the structure of an example of an air core coil used for an NMR imaging apparatus. Four air core coil assemblies CS1, CS2, CL1 and CL2 are arranged on the same axis, such that the degree of uniformity of the inner space (within the imaging range) is several tens ppm. Each of the air core coil assemblies comprises a plurality of coil elements in layers and each of the coil elements is formed of a wire 2 wound in the manner shown in FIG. 1.
However, if these coil elements are required to have complete roundness, the degree of uniformity in terms of magnetic field is greatly deteriorated from what is planned. As a means of correcting uniformity, there is a method of, for example, using shim coils, but, this method is inconvenient because various combinations of shim coils are required depending on the desired correction of the degree of uniformity.